1. Field of the Invention
This invention relates to an objective lens for microscope.
2. Description of the Prior Art
In an objective lens for microscope, if NA exceeds 0.3-0.4, the variation in aberration by a parallel planar glass lying between the lens and an object may become significant. Particularly, where the thickness of the parallel planar plate is about 1 mm and is greatly irregular as in a case where observation is effected through the bottom of a "Schale" in the field of tissue culture, aberration fluctuation is great with ordinary lenses and the image may be disturbed. This tendency increases as the NA of the objective lens is increased. A method whereby a suitable lens spacing is selected and such spacing is expanded or contracted by a cam operatively associated with a correction ring to thereby negate the aforementioned aberration fluctuation is known and has been often used. However, it is rare that the aberration fluctuation by the parallel planar plate and the aberration fluctuation caused when the lens spacing is changed negate each other successfully. Therefore, with the conventional objective lens provided with a correction ring, aberrations could not be completely corrected when the thickness of the parallel planar glass fluctuated greatly, and accordingly, the correction range was narrow and aberrations were unsatisfactory in the vicinity of the limit of the correction range. Furthermore, where such objective lens is used as a phase difference objective lens, if the phase difference plate lies rearwardly of the lens spacing (toward the image side), the phase difference plate will deviate from the focus position of the lens when this lens spacing is changed by means of the correction ring. For this reason, the phase ring must be made larger in width to provide for a margin, as a result of which the contrast of the image is reduced. Such disadvantage is unavoidable with the conventional correction system because, in practice, it is almost impossible for the phase difference plate lie forwardly of the lens spacing used for correction (toward the object side).